Image of the Week

 

Missing mass in Albireo Ac: massive star or black hole?

 

Figure 1. The star Albireo, also known as beta Cygni, is perhaps the most popular two-coloured double star in the whole sky. To the naked eye, it is just one star. In a pair of binoculars it resolves into a beautiful pair of orange and blue. At closer scientific scrutiny it further divides into several components – with one of them being a bit of a riddle (red box). The image shows a photo of star clouds in the northern summer Milky Way, covering the well-known constellation Cygnus, the flying swan (orange dashed lines). It includes the star Albireo (below center at right) and sketches its decomposition into several physical parts (coloured boxes). Credits: Milky Way background image: Uwe Reichert, Schwetzingen, Germany. Close-up Albireo A/B photo: Johannes Schedler, Graz, Austria.

 

For about 400 years, roughly since the invention of the telescope in the early 17th century, the bright star Albireo has been known to be a beautiful double. Twentieth-century astrophysics characterized the brighter orange partner (“Albireo A”) as a cool giant star of about five to six solar masses, and the fainter blue one (“Albireo B”) as a hot star of order three to four solar masses.

Already about a hundred years ago, radial-velocity measurements made it clear that the giant Albireo A itself must be a close double. But it took till 1976 to image and measure its very close companion. Thus the giant got renamed Albireo Aa, while the partner got the designation Albireo Ac. Later that partner was characterized as another hot star, a bit less hot and a factor 3 less luminous than long-known Albireo B.

Albireo Ac is too close to Aa to be seen in ordinary telescopic view, even in big instruments. A special technique called speckle interferometry was needed to find it, and to subsequently measure its orbital motion around Aa. By the year 2020, the orbit had been measured sufficiently well to settle its size, shape and period – about 120 years. All observational data – radial velocities, relative motion of the partners Aa and Ac on the sky, their brightness and colours, as well as the distance from the system’s parallax – agreed well within their uncertainties.

Enter the absolute motion of the giant Aa on the sky, as measured by Hipparcos and Gaia, and the neat picture gets completely destroyed. The discrepancy was first noticed in 2018, from just the values of Aa’s proper motions around 1991, 2003 and 2015. They differ much too strongly, but strangely they do so in rough agreement with the established speckle interferometry orbit. In other words: Albireo Ac gravitationally accelerates its bright partner Aa by an exorbitant degree. Either the giant star Aa is very much lighter than reasonably estimated by astrophysicists, or else the partner Ac is much heavier than derived by similar logic.

The former suggestion – Aa being too light – was firmly dismissed in 2019 already, by a careful spectroscopic study of the giant. Just about one year later, a deep analysis of the system - using all available data from radial velocities to speckle interferometry and absolute astrometry - confirmed that the close partner Ac is way too heavy: between 5 and 7 solar masses instead of its astrophysical estimate of somewhat below 3 solar masses.

This riddle is caused by four astrometric quantities: The proper motions from the Hipparcos Catalogue and from Gaia Data Release 2, plus the two positions measured by these two same missions. Either of these four might in principle be corrupted by the unresolved multiplicity of Albireo A. But they would have to be disturbed by an unreasonably large amount to explain the mystery. Also, the Aa proper motion in Gaia’s Data Release 3 fully confirmed - and even strengthened – the finding. Ac still seems much too heavy; very little doubt is left. Nevertheless, an entirely independent astrometric confirmation of the much-too-large acceleration of Albireo Aa would be highly desirable.

That confirmation is available now. It was found in the course of the scientific validation of preliminary double-star solutions using the extended mission time interval to go into Gaia’s Data Release 4 (DR4) in about two years. For the first time, it was possible to firmly measure the acceleration of Aa directly – rather than inferring it from the change of proper motion over an extended time interval. The resulting value grossly disagrees with the assumption of Ac having the astrophysically motivated 2.7 solar masses, and it nicely agrees with the otherwise derived 5-7 solar masses. For the acceleration in east-west direction, this is illustrated in Figure 2, below. This result could not have been achieved using Gaia DR3 data, but it is now possible – by combining the improved data processing we are developing for Gaia DR4 with the extensive additional knowledge that is available in the special case of Albireo.

 

Figure 2: The east-west motion of Albireo Aa as function of time, in units of milli-arcseconds per year. The black dots signify the three astrometric motion measurements mentioned in the text. The black curve is the prediction of that motion from the 2020 combined analysis of the Albireo system which had given the hitherto strongest evidence for the much-too-heavy Albireo Ac. The blue dashed line represents Gaia’s new independent measurement of Aa’s acceleration around the year 2015. It agrees well with the slope of the black curve around 2015, i.e. with the prediction for a high mass of Ac. In contrast, the red line represents the expected acceleration of Aa from the astrophysically expected 3 solar masses for Ac. It clearly deviates from the slope of the black curve. The orange curves indicate possible variants of the black curve which are still compatible with all observational data. Credits: R. Drimmel et al., MNRAS 502, 328–350, 2021

 

Astronomers invariably get excited whenever invisible masses of more than twice the solar mass turn up. They could be black holes. Thus, excitement was high when in November 2022 an international team of astronomers announced the definitive discovery of a black hole from an astrometric binary orbit in Gaia’s Data Release 3 – when combined with other measurements that the team itself had taken. This find was called Gaia BH1 (“Black Hole 1”). It is the closest known black hole ever, being only about 480 Parsec (or 1600 lightyears) from the Sun.

Could the unseen mass in Albireo Ac be another “Gaia Black Hole”? It clearly could be. It would have to be in a very close orbit around the visible star Albireo Ac. If so, it would be the new record holder of most-nearby black hole, as Albireo is only about 100 Parsec (300 lightyears) from the sun. Difficult investigations with a variety of methods and instruments will be needed to definitely prove that. These surely will be undertaken, because the question is really exciting. But, as a caveat, it should be mentioned that there is a second possibility: Albireo Ac could – once again! – be a pair of two stars. These would have to be two very similar hot stars, both just slightly less massive and both just half as luminous as the presently assumed single star Ac. This possibility does not fit the available data well, but it is just barely compatible with them. Again, difficult follow-up work will be needed to confirm or disprove this second variant.

As a funny side remark, it is noted that still another member of the Albireo system has been found last year by a Mexican/German group of astronomers: Albireo Ad is a faint red dwarf star circling the orange giant Aa in a tight orbit every 371 days. However, with its mass of only 0.1 solar, this little guy has absolutely nothing to do with the Aa/Ac mass riddle. And just for historic interest it may also be worthwhile to mention the non-existing system member Albireo Ab: Some decades ago, there was a claim from speckle interferometry data that there is another component. It was named Ab, but it was never seen again. Even if it should exist, it again – like Ad - would have no relevance for mysterious Ac and its mass.

 

Further reading

U. Bastian and R. Anton, The mass of Albireo Aa and the nature of Albireo AB: New aspects from Gaia DR2, Astron. Astrophys. 620L, 2B, 2018

D. Jack et al., Stellar Parameters of Albireo Aa determined with high-resolution spectroscopy, RNAAS 3, 33, 2019

R. Drimmel et al., A celestial matryoshka: dynamical and spectroscopic analysis of the Albireo system, MNRAS 502, 328–350, 2021

D. Jack et al., Yet another star in the Albireo system: The discovery of Albireo Ad, Astron. Astrophys. 661A, 49J, 2022

 

Credits: Story written by Ulrich Bastian on behalf of DPAC’s CU3/astrometry and CU4/non-single stars data reduction teams.

[Published: 09/02/2023]

Image of the Week Archive

2024

22/04: Gaia's contribution to discovering distant worlds

16/04: Gaia spots Milky Way's most massive black hole of stellar origin

02/04: The Gaia Cataclysmic Variable hook

2023

19/12: 10 Science topics to celebrate Gaia's 10 years in space

31/10: Gaia observes cosmic clock inside a heavenly jewel

10/10: Gaia Focused Product Release stories

27/09: Does the Milky Way contain less dark matter than previously thought?

22/09: Mass-luminosity relation from Gaia's binary stars

13/09: Gaia DPAC CU8 seminars

13/06: Gaia's multi-dimensional Milky Way

18/05: Mapping the Milky Way

15/05: Goonhilly station steps in to save Gaia science data

25/04: The Gaia ESA Archive

05/04: Dual quasar found to be hosted by an ongoing galaxy merger at redshift 2.17

21/03: GaiaVari: a citizen science project to help Gaia variability classificaton

09/02: Missing mass in Albireo Ac: massive star or black hole?

31/01: Gaia reaches to the clouds – 3D kinematics of the LMC

25/01: Meet your neighbours: CNS5 - the fifth catalogue of nearby stars

18/01: A single-object visualisation tool for Gaia objects

2022

25/11: 100 months of Gaia data

23/11: The astonishment

09/11: Gamma-Ray Burst detection from Lagrange 2 point by Gaia

04/11: Gaia's first black hole discovery: Gaia BH1

26/10: Are Newton and Einstein in error after all?

21/10: Gaia ESA Archive goes live with third data release

06/10: Mapping the interstellar medium using the Gaia RVS spectra

26/09: Gaia on the hunt for dual quasars and gravitational lenses

23/09: Gaia's observation of relativistic deflection of light close to Jupiter

13/06: Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe

2021

23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4

2020

22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy

2019
20/12: The lost stars of the Hyades
06/12: Do we see a dark-matter like effect in globular clusters?
12/11: Hypervelocity star ejected from a supermassive black hole
17/09: Instrument Development Award
08/08: 30th anniversary of Hipparcos
17/07: Whitehead Eclipse Avoidance Manoeuvre
28/06: Following up on Gaia Solar System Objects
19/06: News from the Gaia Archive
29/05: Spectroscopic variability of emission lines stars with Gaia
24/05: Evidence of new magnetic transitions in late-type stars
03/05: Atmospheric dynamics of AGB stars revealed by Gaia
25/04: Geographic contributions to DPAC
22/04: omega Centauri's lost stars
18/04: 53rd ESLAB symposium "the Gaia universe"
18/02: A river of stars
2018
21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
2017
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
04/10: 1:1 Gaia model arrives at ESAC
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
27/08: Quest for the Sun's siblings
 
Please note: Entries from the period 2003-2010 are available in this PDF document.